When he wrote Mein Kampf in 1924, Hitler was aware that millions of German men had died in the First World War—and millions more wounded. He knew that millions of German men would not pass their genes along—a case of “negative selection.”

He was profoundly disturbed. The best men were the masculine, courageous ones—those who volunteered to fight—and were more likely to die in battle.

The worst men were those who lacked courage—found ways to avoid participating in battle—and were more likely to survive.

As a result, Hitler said, while the “best human material” was being “thinned on the battlefield,” the worst people “wonderfully succeeded in saving themselves.”

This conundrum lay at the heart of Hitler’s thinking: Why do the best (fittest) men die in war, while the worst (least fit) survive?